Fine particles and human health—a review of epidemiological studies
Introduction
Particulate matter (PM) is a mixture of many different components with local and regional variation. PM can be characterised by origin, e.g., anthropogenic or geogenic, primary or secondary particles; by source, e.g., combustion products and traffic, or by physicochemical properties such as solubility. For practical reasons under aspects of immission measurements, PM is characterised by particle size (aerodynamic diameter).
Several metrics have been or are still used. Total suspended particles (TSP) is the most comprehensive term including particles of any size suspended in air. However, particles larger than 30–70 μm only remain suspended for a very short period before deposition. PM10 and PM2.5 is PM with an aerodynamic diameter of less than 10 and 2.5 μm, respectively. Ultrafine particles (UF) are those with a thermodynamic diameter of less than 0.1 μm, also called PM0.1. The PM2.5 fraction is also called “fine particles”, and those particles between 10 and 2.5 μm are currently named “coarse particles”. However, about one decade ago, the term “coarse particles” was used for particles larger than 10 μm. PM10 is also called “inhalable particles”. As shown in Fig. 1, the larger fractions comprise the smaller ones, i.e., PM2.5 is part of PM10, UF are part of PM2.5 and PM10.
Health endpoints often studied in relationship to PM are mortality (total and cause-specific) and morbidity, hospital admissions due to cardiovascular and respiratory diseases, lung function, and functional endpoints, e.g., heart rate variability (HRV).
Commonly used epidemiological approaches are time-series analyses and cohort studies, as reviewed in this paper. Other approaches are case-crossover studies, cross-sectional studies, panel studies, and case-control studies. In practice, these approaches may have some overlap, e.g., cohort studies with nested case-control studies.
Some questions are only in part answered by current knowledge with respect to health effects of PM:
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Which ones are the health-relevant size fractions of PM?
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What about the dose–response relationships (concentration–response relationships) for various endpoints?
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What about effect modifiers?
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Is PM a causative agent, an indicator, a container?
Section snippets
Methodological aspects
Particulate matter is a ubiquitous air pollutant. Health effects concern the whole population. On an individual scale, however, the magnitude of effects on health of ambient PM air pollution is relatively small compared to health effects from smoking (Fig. 2).
The relatively small effect on health of PM can only be adequately examined in epidemiological studies of sufficient size and observation time. Data needed for large time-series studies on mortality are generally easily available. In
Time trend and revised studies
During the last decade, effect estimates from time-series analyses showed a downward trend. A compilation published in 1994, based on 10 time-series studies performed in the US, provides an RR estimate of 1.06 associated with a 100 μg/m3 increase in TSP (Schwartz, 1994). Supposing a relation PM10=0.6×TSP, the indicated RR corresponds to an RR of 1.01 for a 10 μg/m3 increase in PM10 concentration. The summary of relative risk estimates given in the WHO Air Quality Guidelines for Europe (WHO, 2000)
Fine or coarse particles
Recently, a WHO Working Group stated: “There is strong evidence to conclude that fine particles (<2.5 μm, PM2.5) are more hazardous than larger ones (coarse particles) in terms of mortality and cardiovascular and respiratory endpoints in panel studies. This does not imply that the coarse fraction of PM10 is innocuous”. Another statement was “… the relative importance of fine and coarse PM may depend on specific sources present in some areas but not others” (WHO, 2003).
In many studies, PM
Health endpoints and effect modifiers
In time-series studies, cardiovascular and—as far as studied—respiratory effects seem to be relatively greater than effects on other endpoints (cf. Table 2, hospital admissions). Total mortality is less prone to misclassification than cause-specific mortality. In the Six Cities Study and the ACS cohort, respiratory deaths were not associated with PM2.5 (HEI, 2000). However, respiratory deaths are a relatively small fraction, and the separation from cardiovascular deaths may be incomplete.
Concluding remarks
The body of epidemiological literature on PM health effects is impressive. A review necessarily has to focus on a limited choice of aspects. The role of particle size is an important issue. For air pollution control, it would be helpful to know exactly which part of PM is the fraction most relevant to health. Epidemiology can demonstrate associations between all PM fractions and health endpoints. No single study alone can prove or refute a relationship with one or another fraction. It seems to
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